Rankings were based on average points per tournament when I started following tennis around '83/'84. Don't know specific numbers but some of the concepts used were:

* A minimum divisor of 12 was used, so if you played < 12 events your total points were still divided by 12 to normalize the average totals. (So a player wouldn't be able to game the system by playing only the Grand Slam events then not any of the smaller events.)
* Players earned points for the rounds they reached in an event, as well as "quality" points based on the ranking of the opponents she beat. The WTA eliminated the quality points a few years ago.
* For a while, the computer used a system of diminishing returns, where the recent 6 months were weighted more than the prior 6 months. That's partially why Evert was able to recapture #1 from Navratilova after winning the '85 French - Evert had taken the last two majors ('84 Australian in Dec '84, and '85 French in June '85), and the points for her Slam titles were at full weight. Navratilova had the other two slams ('84 Wimbledon & US) but didn't have full weight for those events.

The WTA did away with the averaging system in the mid/late 90s. I believe it was to encourage players to play more - you can now continue to build your ranking points by playing more events. The downside of this is that all of a player's losses may not count against her if she plays more than 18 events (witness Bartoli has lost >30 times in '07, but still ranks among the top ten), and injuries may be more prevalent due to a player's overplaying.

I am so sorry but I don't remember. Some time in the last 5 years the divisor was changed but I can't recall the details. Somebody here should know. I think the reason why I couldn't retain the rule change is because it became so convoluted.

Simply put, the WTA wanted its top stars to play more often. Compared to her immediate predecessor at #1, Steffi Graf tended to play a fairly limited schedule even during her early years at the top of the women's game and she started to trim her schedule down even more when she started suffering from serious injuries. For instance, Navratilova and Evert played each other 12 times in 1983 and 1984 whereas Graf met Seles only once in all of 1996. I also suspect that the WTA found it a bit embarrassing that Graf was able to comfortably hold on to her #1 ranking in spite of playing only 11 events each in 1995 and 1996, which was below the minimum divisor in both years - 12 in 1995 and 14 in 1996.

As a result the WTA officials decided that switching to an additive ranking system made good business sense as they felt it would force the top players to play more often. They put the matter to a vote and the rank-and-file players mostly supported the move. The top players (Graf, Seles, Novotna) opposed the proposal and Graf went so far as to state that she wasn't going to play any more than she already was, no matter what ranking system the WTA used. Ultimately, I think this decision cost the WTA as it mostly resulted in more and more players suffering serious injuries due to overplaying and many more ranking controversies than had ever been under the old divisor.

Best left-right combination by a German (and that includes Max Schmeling): Steffi Graf. All she did in 1987 was knock Navratilova out of #1 and try to knock Evert out of the sport. (Mike Lupica in "The Best and Worst of Tennis in 1987", World Tennis)

"A couple of years ago, we nicknamed Steffi Graf's forehand 'Jaws'. And that music would go perfectly when she starts running in to the net, swarming on that little ball." (JoAnne Russell, during the 1988 Wimbledon final between Graf and Navratilova)